This invention relates to an improved method of manufacturing a polyvinyl alcohol (PVA) film.
When manufacturing a plastics film, it is subjected to elongation, usually in the longitudinal and transverse directions for the purpose of improving the physical characteristic of the film. As a consequence, it is possible to reduce the thickness of the film to a minimum value required for packing or other purposes and to increase the area and strength thus saving the cost. Such two axis elongations can be performed by simultaneously or successively elongating the film in the longitudinal and lateral directions.
Films of polymers such as stylene, polypropylene, polyester, etc., are generally subjected to successive elongations and manufactured in a large scale. However, when a polyamide film is elongated successively along two axes the film tends to crystallize greatly during the longitudinal elongation and strong hydrogen bonds are formed by amide radicals. Such crystallization and bonding make it difficult to elongate. For this reason, the successive two axis elongation process is not suitable for such polyamides and simultaneous two axis elongation process as disclosed in Japanese utility model publication No. 17197 of 1963 has been actually used in which a pantograph type link mechanism including clips for clamping the edges of the film is used. Similar to polyamides polyvinyl alcohols also contain hydroxyl groups in the molecular chains so that strong hydrogen bonds are formed as the crystallization proceeds due to elongation. Accordingly, it is extremely difficult to apply the successive two axis elongation process to PVA and the simultaneous two axis elongation process has been used.
However, a simultaneous two axis elongating machine is constructed such that the side edges of a film are clamped by a plurality of clips, that the longitudinal spacing between adjacent clips are increased during running to elongate the film in the longitudinal direction and that simultaneously the clips are moved in the lateral direction to elongate the film in the lateral direction. Accordingly, such machine is not only complicated and expensive but also difficult to operate and maintain. Moreover, as the pantograph type endless link mechanism for controlling the movement of the clips is elaborate and complicated it is not suitable for high speed running. Thus, its production speed is low. Accordingly, such machine cannot be used economically for the preparation of thin PVA films resembling cellophane films, and it has long been desired to provide an improved successive two axis elongation process that can use an inexpensive roller type longitudinal elongating machine and a clip type lateral elongating machine having high production speed.
When manufacturing PVA films, it is difficult to manufacture them at high speeds without elongation so that semi-molten method is now used most widely because of its high production speed wherein PVA is dissolved in water at about 140.degree. C. to form an aqueous solution having a concentration of about 50%, the solution is then caused to flow onto a roller or a belt through a narrow slit and then dried to form a film. This method, however, involves such steps as transferring, peeling and drying which hinder high production speed. Thus, the production speed is limited by a slow drying speed for thick films and difficulty in the transfer and peeling of thin films. Such problems increase the cost of the PVA films. Yet due to their low tendency of being charged with electric charge, and good printing property they are formed as relatively thick films by incorporating a suitable amount of a plasticizer and without subjecting to any elongation treatment, such thick films having been used to pack fabrics or other fiber products. PVA films have the highest resistance against oxygen permeation among various plastics so that they are suitble for packing fragrant things. In addition, they are highly transparent and have good printing property. For this reason, when subjected to two axis elongation process, the PVA films increase transparency and have high mechanical characteristics, especially increased stiffness. Accordingly, even when their thickness is decreased they can be used as an efficient packing material. High moisture absorving property, the most serious defect of the PVA films intended for use as packing material, can be prevented by applying a thin coating of a water resistant polymer on the surface of the PVA films.
PVA films prepared by simultaneous two axis elongation process have well balanced physical properties which is highly evaluated in the art. If the simultaneous two axis elongation process could be applied for the production of this PVA films at high efficiency this process would be the best. In any event, it has been desired to develope an efficient method capable of economically preparing thin two axis elongated PVA films not containing or containing a small amount of plasticizer suitable for use as packing material.